A conventional injection molding machine usually comprises a machine base, a clamping unit, an injection unit and an electro-controlled hydraulic system; the present invention is merely related to that injection unit.
The conventional injection process includes the steps as shown in FIGS. 1--1 to 1-4; such steps include damping mold, injection and holding pressure, cooling and metering, and demolding. The plastic material in hopper "A" can automatically fall into a heating barrel "B"; then, the screw "C" turns to mix the material and to convey the material via the screw groove to the front end of the heating cylinder "B". Then the material therein will be melted as result of a heater applying heat to the heating barrel "B" and a shearing heat generated by the rotating screw "C". When as the melted material is conveyed to the front end of the heating cylinder "B", a counter pressure will take place to compel the screw "C" to move back to a given position (i.e., the metering position), and then the screw "C" will stop rotating. Subsequently, the hydraulic cylinder "D" of the injection unit will push the screw "C" to move forward to have screw "C" become an injection plug so as to inject the melted material out of a nozzle "E" at the front end of the heating barrel "B" to inject into a mold "F"; after holding pressure, cooling and demolding, and molding product "G" is rolled off the line. The mold "F" may be dosed again to be ready for the next molding cycle. It is apparent that the injecting screw is the important key part of an injection molding machine. It is closely related to the function of conveying, melting, mixing, and metering the material, and the quality control of an end product.
A current conventional screw is shown in FIG. 2, which may be divided, according to its geometrical shape, into three sections, i.e., (1) a feeding section "L1" in which materials are to be pre-heated and conveyed, (2) a compression section "L2" in which the materials are to be compressed with a shearing force, mixed, and pressurized and air-exhausting, (3) a metering section in which the materials are to be mixed, metered and conveyed in a melted state. In real molding process, the functions of the various sections often overlap each other, as it is rather difficult to divide their functions dearly. The features of the screw are that its screw pitches are equal to each other, and its feeding section "L1" has a fixed depth of thread "hF". The depth of thread "hF" is gradually changed from the depth of thread "hF" to a depth of thread "hM" through the compression section "L2". The metering section "L3" has a fired depth of thread "hm"; the depth of thread "hF" is greater than the depth of thread "hM". Most of the ordinary molding factories use one screw to mold different plastic materials, and that screw may be referred to as "universal" screw. In fact, each kind of plastic material has its particular physical characteristics, which require a screw to be designed with different parameters, such as the compression ratio, the length of the three sections and the geometrical figures thereof. In other words, the aforesaid "universal" screw is unable to provide the best molding result. Some factories have become aware of the necessity of using "individual" screws to improve the quality of products. Therefore, they use different screws, which become problems such as higher production costs and management costs.
Currently, a U.S. extrusion machine factory named Leistritz has used a dual screw as shown in FIG. 3-1 to 3-10, in which a hexagonal shaft "Al" is the main member with a given number of other screw elements according to the physical characteristics of a plastic material. The other screw elements include the screw elements A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12 and A13 (as shown in FIGS. 3--3 to 3-10), which are to be sleeved around the hexagonal shaft "Al". A screw head A14 is mounted, by means of threads, at one end of the hexagonal shaft Al and fixes the other screw elements in place.
In fact, the single screw rod used in an injection molding machine is completely different from that used in an extrusion machine. An extrusion machine produces products continuously, and its screw rod rotates continuously in one direction. The screw rod of an injection molding machine rotates discontinuously in one direction and has reciprocating actions. So the motions of an injection screw are sophisticated. The fastening strength of the various screw elements by means of the screw head A14 is much lower. Further, the manufacture of the hexagonal shaft and hole therein is also much difficult. When such a structure is used in an injection molding machine for a screw with a small diameter, the smallest root diameter zones A101, A141 and A142 are susceptible to becoming broken as a result of torsional force. When the screw is rotating and metering, the shearing stress caused by the axial force is much less than by the torsional force. We can suppose in this case that the maximums shearing stress is mainly induced by the pure torsional force and it's value can be estimated as 16T/d, (in which "T" stands for torque, while "d" stands for the cylindrical root diameter). The shearing stress and the torque are in direct proportion, and the shearing stress is in inverse proportion to the cubic value of the diameter of the cylindrical root diameter. Under the condition of not exceeding the diameter zone stress of a material, if the root diameter zone A101 is changed to 0.8 of its original diameter, the torque applied to the screw will be 0.512 times (i.e., 0.8 ) of its original torque. The same is true for the root diameter zones A141 and A142 of the screw head A14; therefore, such structure requires further improvement.